1. Field of the Invention
The present invention relates to a pressure-contact device through which a print head is brought into and out of pressure-contact with a platen of a printer, and specifically to a print head pressure-contact device for applying a thermal head onto a platen by way of a linkage.
2. Description of the Prior Art
Referring now to FIG. 8, there is shown a conventional pressure-contact device of a print head onto a platen. In FIG. 8, the print head pressure-contact device 101 includes a first shaft 102 serving as a drive shaft, a first lever 103 serving as a drive lever and rigidly fixed on the first shaft 102 at one end thereof, a second shaft 104 fixed on the other end of the first lever 103, and a second lever 105 serving as a driven lever and pivotably received by the second shaft 104 at one end thereof. The first shaft 102, the first lever 103, the second shaft 104, and the second lever 105 construct a linkage 108 for applying a print head 109 onto a platen 110. The second lever 105 has a third shaft 106 firmly fixed at the other end thereof. The third shaft 106 pivotably receives a print head pressure-contact plate 107. The head pressure-contact plate 107 is pivotably received on a print-head support 111 by way of a pin 115. As seen in FIG. 8, the print head 109 is mounted on the lower surface of the head support 111. The head support 111 is pivotably received by means of a stationary shaft 112 firmly fixed onto a printer housing (not shown). A spring 113 such as a coil spring is arranged between the head pressure-contact plate 107 and the head support 111, for pressing the head 109 onto the outer periphery of the platen 110 with a desired pressure. Under the condition in which the head 109 is out of contact with the platen 110, the head support 111 is kept in a spring set position wherein the end of the support 111 abuts a flanged stopper 114 integrally formed with the head pressure-contact plate 107. The first shaft 102 has a driven connection with a driving system which generally includes an electric motor (not shown) and a reduction gear set both mounted on the printer housing (not shown).
With the above arrangement, torque generated by the drive motor is transmitted from the first shaft 102 to the first lever 103. In accordance with rotational movement of the first lever 103, the second lever 105 is extended and bent with respect to the first lever 103, with the result that the head pressure-contact plate 107 and the platen 110 are movable towards and away from each other. Accordingly, the head 109 is movable in such a manner as to be brought into pressure-contact with and out of contact with the outer periphery of the platen 110 in accordance with the rotational movement of the first lever 103. In the previously-noted conventional head pressure-contact device with the linkage 108 constructed by the two levers 103 and 105 coupled rotatably to each other, there is a drawback as described hereinbelow described in detail.
Supposing that the first shaft 102 corresponds to an engine crankshaft, the first driving lever corresponds to a crank, the second shaft 104 corresponds to a crankpin, the second driven lever 105 corresponds to a connecting rod, and the third shaft 106 corresponds to a piston pin, the third shaft 106 has a lower limit point (viewing FIG. 8) similar to a top dead center generally abbreviated as "TDC" in the case of reciprocating engines, when the two levers 103 and 105 are aligned with each other. Although the lower limit point of the third shaft 106 must be univocally determined, however, the lower limit point tends to fluctuate owing to errors in machining accuracy of the first and second levers 103 and 105, several deviations, namely a deviation from an ideal connecting point of the first shaft 102 to the first lever 103, an eccentricity of an ideal bearing hole of the second lever 105 rotatably coupled with the second shaft 104, and a deviation from an ideal mounting point of the third shaft 106 on the second lever 105, or owing to a designated backlash or play in the driving system consisting of a drive motor and a reduction gear set. Thus, such a conventional head pressure-contact device requires a high machining accuracy. When the head 109 is pressed onto the platen 110, a reaction force is created to act on the drive shaft 102 through the links 103 and 105, owing to a pressure of the head 109 pressed onto the platen. Across the TDC of high degree of freedom as appreciated from a free movement of the levers rightwards and leftwards at the TDC, a direction of moment of the reaction acting on the first shaft 102 in a lever position as indicated by a solid line of FIG. 8 before the third shaft 106 reaches to the TDC becomes opposite to a direction of moment of the reaction acting on the first shaft 102 in a lever position as indicated by a two-dotted line of FIG. 8 after the third shaft 106 has reached the TDC. That is to say, the linkage 108 exhibits an excessively unstable force balance in the vicinity of the TDC. Therefore, there is a tendency for the pair of levers 103 and 105 easily to escape out of the TDC owing to backlash of the reduction gear of the driving system, upon the third shaft 106 has reached to the TDC. Furthermore, the life of the motor is shortened due to the above-mentioned opposite reaction moment acting on the driven shaft. In order to avoid this, it is necessary to precisely stop the pair of levers 103 and 105 in a designated position in which the third shaft 106 does not yet reach to the TDC. Thus, the conventional head pressure-contact device requires a high-accuracy control for the driving system.